Scientists Discover the Secret to a Cavefish’s Weird, Waddling Walk

When Daphne Soares got back from a trip to Thailand last year, she had something she just had to share with her hallmate at work. “Check out this cool fish I saw!” she said, pulling up a video. Brooke Flammang had never seen anything like the cave fish in Soares’ footage. It was pink. It was eyeless. It had huge fins that looked like two pairs of wings. And it was walking.

But it wasn’t the walking that threw her for a loop. Flammang studies fish locomotion at the New Jersey Institute of Technology, so she’s used to seeing fish moving on land. She wasn’t surprised to see one that could push itself over rocks and through water gushing like a fire hose. But other “walking” fish hop forward by leaning on their pectoral fins like a pair of crutches, or flex and shimmy to wriggle over surfaces. This one was taking steps, moving one of its front fins in time with the back fin on the other side of its body, alternating in a diagonal two-step like a salamander. Flammang was incredulous. “I was like, ‘Fish can’t do that,’” she says. “That’s ridiculous.”

This one did, though. And a fish that walks like an amphibian, especially when it’s more closely related to a goldfish than to any four-footed creature, could teach biologists a lot about how humans’ fishy ancestors learned how to walk. Intrigued, Flammang asked Soares if she could get a specimen of her own to examine. The answer? “Absolutely not.”

It turns out the waterfall-climbing cave fish Cryptotora thamicola is incredibly rare. It lives in just eight caves on the border of Thailand and Myanmar, and the Thai government fiercely protects the small population—fewer than 2,000 adults at last count.

Danté Fenolio/Science Source

Normally, a biologist studying anatomy and movement would capture some fish from the wild, film how they move in a lab, and dissect a few to figure out how their bones and muscles work together. No such luck with Cryptotora thamicola. If Flammang was going to get a better look at how this fish walked, she was going to have to do it inside the caves where they lived. She would have to study its skeleton without dissecting a single specimen.

Only a few years ago, Flammang would’ve been stuck. That awesome video would probably have been all she knew about this novel cavefish. Because even if she could get permission to film in the caves, she would still need a way to see what the fish’s skeleton looked like. And none of the museums that owned preserved specimens were about to let anyone cut into something that rare. “Museum specimens are everybody’s specimens,” says Paul Gignac, a biologist and 3-D imaging specialist at Oklahoma State University. “They’re not things you can sample destructively, especially when they’re very rare specimens that other people may someday need.”

Fortunately, Flammang knew something she could do.

She started by teaming up with a Thai ichthyologist, Apinun Suvarnaraksha, who was able to get permission to go into the caves and capture fish to film before rereleasing them into the wild. Suvarnaraksha had never collected kinematic data before, so Flammang trained him remotely. “I tried to give him the most explicit instructions I could, then he’d collect a few videos, upload them on Google Drive, and send them to me,” Flammang said. “And I’d notice that we needed a better camera angle, or better backlighting. It went back-and-forth for a couple of months until we got some videos to analyze.”

Brooke Flammang

Suvarnaraksha also got permission to scan a museum specimen in a high-resolution CT scanner at a local dental school, which gave Flammang the data to build a detailed 3-D model of the fish’s skeleton while leaving the specimen unharmed.

Today, they released their results. The videos confirmed that Cryptotora thamicola moves a lot like a salamander, keeping its tail straight as it steps its fins forward, utterly unlike the typical wriggly fish-out-of-water. Their model of its skeleton, published in Scientific Reports, hints at how the fish manages that feat: Its pelvis is fused to its vertebral column, which lets the fish push forces from its limbs directly into its core.

Flammang thinks that anatomy is a first for modern fish, though it’s common in terrestrial vertebrates. “When they sent me the files, I thought someone was playing a trick on me,” she says. “There was this gigantic pelvis that looks nothing like any fish pelvis.”

It’s a neat bit of convergent evolution that wouldn’t have been discovered before high-resolution consumer camcorders, widespread CT scanners, and remote collaboration via the Internet. Gignac notes that Flammang’s approach is an effective way to pull new data from rare specimens around the world. “CT lets us nondestructively get a much better understanding of their anatomy than we could from just photographing them at a museum. And one of the most important advances it’s given researchers is an ability to work collaboratively in groups all over the world.”

A CT scan isn’t quite like holding a fish in your own two hands, but for Flammang, it’s better than nothing. Way better.

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